This invention relates to lensed optical systems and more particularly to an optical lens having reduced veiling glare. A lensed optical system is a system containing optics that transmit light rather than reflect light.
All optical elements contribute to stray light or "veiling glare". Stray light is any unwanted unimaged light received by a detector in an optical system. In a lensed optical system, stray light can reflect off of the multiple lens elements causing severe imaging problems by decreasing visibility of low contrast objects. For example, in image intensifier tubes, it results in a loss of contrast by filling in the darker portions of the image.
Prior methods of minimizing stray light have been directed to the baffles and surfaces of the optical components and in particular to the outer cylindrical surfaces or edges of the lenses themselves. To reduce scattering from the lens surfaces, the surfaces are usually left with a ground finish and is painted or stained with some sort of ink or enamel. Felt tip markers are frequently used.
These methods have been based on the assumption that photons striking the lens surface will be transmitted into the ink and absorbed. Unfortunately, it has been found that this does not always occur. Minimum reflection amplitude off the glass/ink interface is approximately 4% for normal incidence depending on the glass index of refraction. This is typical Fresnel reflection which does not account for scattering effects. Two parameters increase this reflection amplitude. The first parameter is geometry. Almost all photons impinge the lens edge surface at angles other than normal incidence so that the Fresnel reflection amplitude is higher than 4%; for rays striking the interface at angles greater than the critical angle the reflection becomes 100% off the glass/ink interface. Scattering of photons off of the microstructure, cracks and other defects on the lens surface on an atomic level is the other parameter that will increase the reflection amplitude. Where the interface is ground glass/ink, the reflection amplitude for normal incidence will rise from 4% to approximately 10%.
Another problem arises due to the fact that most inks are highly transmissive in the 600 nm to 1000 nm (red) spectral region. Since absorption is significantly lower than expected, photons are actually transmitted to the mechanical housing in which the lens is mounted, such as a lens barrel, and are scattered throughout the optical system. This is a significant problem for optical systems which operate outside of the 400 nm-700 nm (photo-visual) spectral range, such as image intensifiers which work in the red spectrum.
It is therefore an object of the present invention to provide a optical system having reduced light scatter.
It is an addition object of the invention to provide a lens for reducing the incidence of stray light in an optical system.
It is a further object of the invention to provide a method for forming such an optical lens in an easy and economical manner.
These objects and others which will become apparent hereinafter are accomplished by the present invention which provides a lens formed of an optical material. The outer surface of the lens has one portion for receiving light, another portion for transmitting light and the remaining portion for absorbing stray light. The absorbing portion includes optical silica material and a reduced metal oxide which forms a blackened area in the lens.
The present invention also provides a method of reducing stray light in an optical lens by forming a blackened area in the outer surface of an optical material. The blackened area is produced by reducing a metal oxide of the optical material in the outer portion of the lens with hydrogen gas at an elevated temperature and pressure. The reduction causes the oxygen to escape from the metal oxide, leaving the metal oxide in a reduced form to remain in the outer surface of the optical material.
Additional insight into the present invention may be obtained by reference to commonly owned, copending U.S. patent application entitled "Cathode for Image Intensifier Tube Having Reduced Veiling Glare", Ser. No. 233,501, and filed 8/18/88, now U.S. Pat. No. 4,961,025.